Barnes, ousted after just one term, made the announcement surrounded by family at a news conference today. The Democrat pledged to create jobs in Georgia and to improve education and transportation. He acknowledged past mistakes, such as feuds with the state's teachers, that some have blamed for his surprising defeat in 2002.
"I realize that when I was governor before I didn't do enough listening. I realize that I was impatient and had an aggressive agenda," Barnes said.
"My mama said I was the hard-headedest kid that God ever put on the face of the earth and my mama was always right. But I did learn from those experiences and I learned from my mistakes."
Barnes said he will not file paperwork to launch his campaign officially until July, so that he can complete a term as chairman of the National Board for Professional Teaching Standards.
The addition of Barnes is sure to shake-up the 2010 governor's race, which is already in full swing with more than a year to go in the campaign. Several political insiders expect Barnes will become the front-runner for the Democratic nomination.
Barnes was booted from the governor's mansion in 2002 by Republican Sonny Perdue, then a little-known state senator. In recent months, the 61-year-old lawyer has been publicly flirting with a comeback.
Three Democrats already in the race are Attorney General Thurbert Baker, House Minority Leader DuBose Porter and former Georgia National Guard Commander David Poythress. Six Republicans are also running.
Barnes has been biding his time at the helm at of his lucrative law practice since losing his re-election bid.
In recent months, he has been turning up at prayer breakfasts and chicken dinners around Georgia, road testing a populist message that often sounds a lot like a stump speech.
Last month, Barnes said he was concerned that special interests were setting the agenda in the Legislature and "the priorities they are really establishing they destroy the public schools, they forget about transportation, we have no concern about our water problems because now its raining I think are all the wrong priorities of the state."
Barnes, at the time, said he was struggling with whether to try his hand at another campaign.
"It's a difficult decision because I never planned to come back. I mean I was completely satisfied with my life," Barnes said on May 12. "However as I sit around and see, now, soon to be six grandchildren, what kind of life are they going to have in this state and future?"
Whether Barnes can carry Georgia's struggling Democrats is still an open question.
Some in the Democratic Party remember him as the polarizing figure who lost to Perdue, a rural state senator who ran on a shoestring budget. Barnes' loss opened the door to the first Republican governor in Georgia since Reconstruction and ushered in GOP rule in the state.
He had a reputation for arrogance that spawned an infamous 2002 campaign ad depicting him as a rat named "King Roy." He feuded with the state's teachers over education reform.
And for all his talk of the corrupting influence of lobbyists, Barnes was once criticized for being beholden to the state's business and development interests.
Still, he has earned some recognition that could help him rehabilitate his image since his loss.
He received a John F. Kennedy "Profile in Courage" award in 2003 for his work to remove the Confederate Battle symbol from the Georgia flag.
And he strikes a humbler tone these days.
At the Democrats' annual fundraising dinner last month, Barnes said the current field lacked vision.
"It's not that they're not good people. It's just that, listen, we can do better," he said outside the Democratic dinner.
Barnes is a formidable fundraiser he scooped up $20 million in his 2002 race against Perdue and could make it difficult for other Democrats to compete.